I hope you got a helmet. After that it kind of depends on what you want to do. In no particular order.

I can't stand to not have mileage and HR data for my log, so for me that was an essential purchase.

Water bottles and cages

Clipless pedals

Tubes and a pump, some tools, a bag

Lights if you plan to be out after dark or could get caught after dark.

Gloves

Happy riding!

V.